The present invention is directed to an improved process for producing metal castings using the lost foam casting process.
Lost Foam Casting (Full Mold Casting) involves placing a plastic pattern of the desired cast part in sand and then pouring molten metal onto the plastic casting causing it to vaporize. The molten metal exactly reproduces the plastic pattern to provide the ultimate casting. Many patents have issued covering the Lost Foam Casting process.
It is known that polystyrene, the major polymer used in this application, produces surface defects when casting iron due to carbon residues left by the polymer. When casting low carbon steel the carbon formed from the polystyrene dissolves in the metal degrading the properties of the cast part. A number of patents describe variations in the Lost Foam Casting process that are intended to minimize the residues left by the polymer after the metal has been poured. Most of these variations involve changing the coating on the pattern or changing the flask in which the casting is made. For example, U.S. Pat. Nos. 4,448,235 and 4,482,000 describe a variable permeability casting designed to avoid entrapment of polymer vapors in the casting. U.S. Pat. No. 3,572,421 describes a flask containing many air breathing holes to allow the escape of polymer degradation products to decrease the formation of carbon. Similarly, U.S. Pat. Nos. 3,842,899, 3,861,447 and 4,612,968 describe the addition of vacuum to the casting flask to aid in the removal of the polymer residues.
The Dow Chemical Company has reported the development of a polymethyl methacrylate foam bead useful to replace polystyrene for the casting process. (Moll and Johnson, "Eliminate the Lustrous Carbon Defect With New Moldable Foam", Evaporative Foam Casting Technology II Conference, Nov. 12-13, 1986, Rosemont, Ill.). Although this polymer reduces residues left on the cast part, it carries with it other disadvantages. The higher glass transition temperature (130.degree. C.) of the polymer causes longer molding cycles when preparing patterns. It uses a Freon blowing agent which has been shown to cause corrosion of molds. It also rapidly gives off a large volume of gas when castings are made. It is very difficult to control the evolution of gas and often the molten metal is blown back out of the flask.
There is still a great need for a polymer that provides the advantages of polystyrene but produces no carbon defects. U.S. Pat. Nos. 4,773,466 and 4,763,715 teach to use polycarbonate copolymers and terpolymers, respectively, to make patterns for the lost foam casting process.